Grating structure for dividing light

ABSTRACT

A grating structure and a solar cell assembly. In one aspect, the grating structure suppresses the zero order transmission to near 0%. In another aspect, the solar cell assembly has improved absorption due to coupling with a grating structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No. 12/638,334filed Dec. 15, 2009, the content of which is hereby incorporated byreference in its entirety.

BACKGROUND

1. Field of the Invention

The application generally relates to a grating structure for dividinglight.

2. Description of Related Art

Various grating structures have been introduced in industry. Gratingstypically have rows of grating lines that diffract light. The diffractedlight is generally distributed into a diffraction pattern forming anumber of diffraction orders. One type of diffraction grating is atransmission grating. Typically, transmission gratings comprise groovesetched into a transparent material. As the elements of light in theincident spectrum strike the grooves, they are diffracted and,therefore, separated to various degrees.

SUMMARY

One aspect of the application provides an improved grating structure forsplitting light.

In solar cell applications, the absorption layer has the highestabsorption efficiency when light travels horizontally or in plane withthe absorption layer. To facilitate horizontal travel of the light thatis received normal to the absorption layer, a grating may be used inconjunction with the absorption layer to diffract the light to be morein plane with the absorption layer. However, for a typical transmissiongrating most of the light travels through the grating in the zero ordertransmission. As such, a grating can be designed to suppress the zeroorder transmission, thereby redirecting the light energy to the first orhigher order transmission at higher diffraction angles.

In one configuration, the grating structure includes alternating ridgesand grooves. The ridges and grooves are configured such that the angleof the first order transmission is at least 40 degrees, for exampleabout 50 degrees.

In another aspect, the zero mode amplitude contribution and the firstmode amplitude contribution may be approximately the same magnitude and180 degrees out of phase.

Further objects, features and advantages of this invention will becomereadily apparent to persons skilled in the art after a review of thefollowing description, with reference to the drawings and claims thatare appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a transmission grating;

FIG. 2 is a perspective view of a transmission grating;

FIGS. 3a and 3b are flow charts illustrating a method for producing agrating;

FIG. 4 is a graph illustrating effective refractive index for each modeof light with respect to the fill factor of the grating;

FIG. 5 is a graph illustrating the amplitude contribution for each modeto the zero order transmission with respect to the fill factor of thegrating;

FIG. 6 is a graph illustrating the amplitude contribution for each modeto the first order transmission with respect to the fill factor of thegrating;

FIG. 7 is a graph illustrating the amplitude contribution for each modeto the second order transmission with respect to the fill factor of thegrating;

FIG. 8 is a graph illustrating the diffraction efficiency for the zero,first, and second order transmission with respect to the grating height;

FIG. 9 is a graph illustrating the amplitude contribution for the zeromode and first mode to the zero transmission with respect to the angleof the first order transmission;

FIG. 10 is a graph illustrating the diffraction efficiency for the zero,first, and second order transmission with respect to the angle of thefirst order transmission;

FIG. 11 is a graph illustrating the difference of amplitude contributionto the zero order transmission between the zero mode and the first orbetween the zero mode and second mode with respect to the fill factor ofthe grating;

FIG. 12 is a graph illustrating the effective refractive indexdifference and the fill factor of the grating;

FIG. 13 is a graph illustrating the diffraction efficiency of eachdiffraction order for s-polarization and p-polarization with respect tograting height;

FIG. 14 is a side view of one embodiment of a grating having a 40 degreefirst order transmission;

FIG. 15 is a side view of one embodiment of a grating having a 50 degreefirst order transmission;

FIG. 16 is a graph illustrating the amplitude contribution for each modeto the zero order transmission with respect to refractive index;

FIG. 17 is a graph illustrating the diffraction efficiency for the zeroand first order transmission with respect to refractive index;

FIG. 18 is a graph illustrating the diffraction efficiency for the zeroand first order transmission with respect to grating height for thegrating in FIG. 14;

FIG. 19 is a graph illustrating the diffraction efficiency for the zeroand first order transmission with respect to grating height for thegrating in FIG. 15;

FIG. 20 is a side view of a solar cell assembly;

FIG. 21 is a graph illustrating the transmittance for s-polarization andp-polarization with respect to normalized wavelength.

FIG. 22 is a graph illustrating the absorption for s-polarization andp-polarization with respect to normalized wavelength.

FIG. 23 is a side view of a solar cell assembly illustrating themagnetic field for s-polarization;

FIG. 24 is a side view of a solar cell assembly including a grating anda reflector;

FIG. 25 is a graph illustrating the transmittance with respect tonormalized wavelength;

FIG. 26 is a graph illustrating the absorption with respect tonormalized wavelength;

FIG. 27 is a side view of a coupler; and

FIG. 28 is a -side view of a coupler illustrating the wave propagationof the light.

DETAILED DESCRIPTION

Referring to FIG. 1, a system 10 including a transmission grating 11 isprovided. The transmission grating 11 may be a fused silica transmissiongrating with a silica to air interface. As such, air surrounds a silicastructure 12 and is denoted by reference number 14. The silica structure12 includes a base 16 of solid fused silica. Fused silica is verytransparent and transmits a very broad bandwidth of light. Further,fused silica offers a very stable material that can be used over a widerange of temperature conditions. In addition, fused silica gratings maybe easily etched to provide the grating properties required for manyapplications. Fused silica has an index of refraction of about 1.45 incontrast to air with an index of refraction of about 1. The symbol n_(α)is used to denote the refraction index of air and n_(β) is used todenote the refraction index for fused silica. For the gratings andapplications described herein, the incident light considered isgenerally in the range of λ=350 nm to 1600 nm. However, particular casesmay be considered for enhancement of blue, green, and red colors, atλ=450 nm, 550 nm, and 700 nm, or for coupling applications at λ=1550 nm.

Protrusions 18 extend from and are integral with the base 16. Beingintegral with the base 16 the protrusions 18 are also formed of fusedsilica. The protrusions 18 form grooves 20 located between eachprotrusion 18. The grooves 20 may be filled with air 14, therebyproviding an air fused silica interface across the grating layer 22. Thegrating layer 22 diffracts light directed towards the transmissiongrating 11 from a light source into various diffraction orders. Each ofthe protrusions 18 may form a ridge 39 that extends to provide a uniformline structure, as denoted by lines 40 in FIG. 2. The protrusions 18 mayhave a top surface 42 and side surfaces 44. The side surfaces 44 mayhave various profiles or may be substantially straight and form a rightangle with the top surfaces 42.

Referring again to FIG. 1, light may be provided to the transmissiongrating 11, as denoted by arrow 30. For the systems described herein,the light 30 has an angle that is substantially parallel to theprinciple axis 31 of the grating projections 18. In addition, theincident light 30 may comprise various light polarizations. For example,the incident light may comprise components that are s-polarized 30A andcomponents that are p-polarized 30B. S-polarization denotes when theelectrical field is perpendicular to the plane of light propagation.P-polarization denotes where the electrical field is parallel to theplane of light propagation. When the incident light 30 interacts withthe grating layer 22, the incident light 30 will form reflectivecomponents denoted by R and transmissive components denoted by T.

The reflective components may form a diffraction pattern comprised of aplurality of diffraction orders. For example, the zero order diffractionof the reflective component R_(n=0) is denoted by arrow 32. Similarly,the first order diffraction of the reflective component R_(n=1) isdenoted by arrow 33 and the −1^(st) order diffraction of the reflectivecomponent R_(n=−1) is denoted by arrow 34. The angle for the −1^(st)order diffraction is θ_(r,−1), while the angle for the first orderdiffraction is θ_(r,1).

The transmissive components may also form a diffraction patterncomprised of a plurality of modes. For example, the zero orderdiffraction of the transmissive component T_(n=0) is denoted by arrow36. Similarly, the first order diffraction of the transmissive componentT_(n=1) is denoted by line 37 and the −1^(st) order diffraction of thetransmissive component T_(n=−1) is denoted by line 38. The second orderdiffraction of the transmissive component T_(n=2) is denoted by line 23and the −2^(nd) order diffraction of the transmissive component T_(n=−2)is denoted by line 25. The angle for the −1^(st) order diffraction isθ_(t,−1), while the angle for the 1^(st) order diffraction is θ_(t,1).Similarly, the angle for the −2^(nd) order diffraction is θ_(t,−2),while the angle for the 2^(nd) order diffraction is θ_(t,2).

The resulting characteristics of the reflective and transmissivecomponents are a factor of the refractive index (n) of the material, theperiod (p) of the grating, the fill factor (r) of the grating, and theheight (h) of the grating. The period of the grating is the distancefrom the start of one groove to the start of the next groove. The periodof the transmission grating 11 is denoted by reference numeral 24. Thefill factor (r) can be defined as the ratio of the ridge width or groovewidth to the period of the grating, which is denoted by referencenumeral 26. The height (h) of the grating is the distance from the topof the protrusion 18 to the bottom of the groove 20, which is denoted byreference numeral 28 in FIG. 1. In one embodiment, the grating may be arectangular grating having rectangular grooves and ridges. However onewould readily understand, that in other embodiments the grooves 20 andprotrusions 18 may not form exact right angles and various profiles maybe used along the edge 42 of the protrusions 18. As such, the definitionfor the fill factor (r) or grating height (h) may be slightly modifieddepending on the shape of the projections 18 and grooves 20. As such,these values may be determined based on the center of gravity of theprojections 18 and grooves 20.

The light path formed by the transmission grating 11 having an interfaceof air/fused silica may be analyzed by modal analysis. The modalanalysis can be derived to provide simultaneously a high efficiency forthe +/−1st order transmission and suppressed transmission to the zeroorder diffraction. The amplitude contribution of each mode to the zeroorder transmission can be used to select the fill factor of the grating.The diffraction efficiency of each diffraction order may then be used toselect the groove height. The grating structure fulfilling thiscondition may exhibit a transmittance for normal light above 90% at anangle of more than 40°.

With regard to analysis methods, rigorous coupled-wave analysis has anadvantage of accommodating various groove shapes. Several shapes ofgrooves such as semi-circle, rectangular, triangular, and curvedsurfaces can be used. Coupled-wave analysis is typically used fordesigning gratings, but due to various assumptions this method would notidentify the described parameters. Coupled-wave analysis is a numericalanalysis and does anticipate propagation mode and evanescent modeintegration. On the other hand, a modal analysis can provide a physicalinsight of diffraction phenomena, although it has less flexibility toadapt for various groove shapes.

When gratings are used for unpolarized light such as light emittingdiodes (LEDs), both p-polarization and s-polarization should besimultaneously taken into account in the design. Particularly, the useof −1^(st) and 1^(st) order transmission extends the design degrees offreedom for optical devices, components, and assembled systems due tolarge bending of light for trapping light in the substrate.

However according to the method described herein, a rectangular gratingmay be derived through a modal analysis to provide simultaneously a hightransmittance for the first order transmission and a very lowtransmittance for the zero order transmission. The analysis can identifya fill factor that produces a substantially equal amplitude contributionto the zero order transmission by the zero mode and another mode. Thisallows a grating height to be selected that produces a cancelling effectbetween the zero mode and the other mode with a substantially equalamplitude contribution.

The grating 11 may be coupled to an absorption layer to form a solarcell assembly. As such, the grating 11 may redirect the light topropagate horizontally within the absorption layer to increaseabsorption. Further, another grating having similar properties tograting 11 may be coupled to the opposite side of the absorption layerthereby redirecting light transmitted toward the opposite side of theabsorption layer horizontally within the absorption layer.

Referring to FIG. 3a , a method for producing a grating according to oneembodiment is provided in process 300. In block 310, the wavelengthrange of light is defined. Further, the angle of incident light isdefined as being normal to the grating. In block 312, the angle ofrequired diffraction angle is defined, for example a first ordertransmission angle 50 degrees. In block 314, the period of the gratingis defined based on the required diffraction angle and the designwavelength. In block 316, the amplitude contribution for each mode tothe zero order transmission is analyzed to identify a fill factor valuewhere the amplitude contribution of the zero mode is about equal to theamplitude contribution of another mode, for example the first mode, asshown in FIG. 5. In block 318, the diffraction efficiency is analyzedfor each diffraction order with respect to grating height at theselected fill factor. This can be used to control the phase relationshipbetween the modes. The grating height is selected that produces theminimum diffraction efficiency in the zero order transmission, as shownin FIG. 8. This generally corresponds to mode 0 and mode 1 being 180degrees out of phase. In block 324, a grating may be fabricated, forexample by etching, based on the parameters determined in the abovenoted steps.

Referring to FIG. 3b , a method for producing a grating according to oneembodiment is provided in process 350. In block 310, the wavelengthrange of light is defined. Further, the angle of incident light isdefined as being normal to the grating. In block 312, the angle ofrequired diffraction angle is defined, for example a first ordertransmission angle 50 degrees. In block 314, the period of the gratingis defined based on the required diffraction angle and the designwavelength. In block 330, the fill factor is selected by analyzing thedifference in amplitude contribution to the zero order transmissionbetween the zero mode and the other modes for both s-polarization andp-polarization. As discussed above, the fill factor may be selected toprovide the same amplitude contribution for the zero mode and firstmode. Then the phase shift between the zero mode and first mode can becontrolled to minimize zero order transmission. As such, a fill factoris selected with a minimum average amplitude difference between the zeromode and another mode (e.g. first mode) for both s-polarization andp-polarization, as shown in FIG. 11. In block 332, the grating height isselected that produces the minimum average diffraction efficiency in thezero order transmission for the s-polarization and p-polarization, asshown in FIG. 13. In block 324, a grating may be fabricated, for exampleby etching, based on the parameters determined in the above noted steps.

Now referring to FIG. 4, a graph is provided of the effective refractiveindex for each mode with respect to the fill factor of the grating. Line410 indicates the effective refractive index for mode 0 based on thefill factor of the grating. Line 412 is the effective refractive indexfor the first mode with respect to the fill factor. Line 414 is theeffective refractive index for the second mode with respect to the fillfactor. Similarly, line 416 is the effective refractive index for thethird mode, while line 418 is the effective refractive index for thefourth mode with respect to the fill factor. Each line indicates thereal portion of the effective refractive index which implies that powercan be transferred through that mode for the given fill factor.

FIG. 5 shows the amplitude for each mode that contributes to the zeroorder transmission according to the fill factor of the grating. Line 510is the zero mode amplitude contribution to the zero order transmissionaccording to the fill factor. Line 512 is the first mode amplitudecontribution to the zero order transmission based on the fill factor.Line 514 is the second mode amplitude contribution to the zero ordertransmission based on the fill factor. Similarly, line 516 is the thirdmode transmission and line 518 is the fourth mode amplitude contributionto the zero order transmission based on the fill factor. In looking atFIG. 5, it is important to understand that regardless of the amplitudefor each mode, the phase of each mode relative to one another can becontrolled by the grating height. Accordingly, if the zero mode andanother mode have the same amplitude value the transmission to the zeroorder (n=0) can be suppressed by selecting a fill factor such that theamplitude of two modes are approximately equal, then the modes can bemanipulated so that they are 180° out of phase. As such, thecontribution from each mode cancels thereby suppressing the amount oflight in the zero order transmission which corresponds to the normaldirection. If the light is suppressed in the zero order transmission, bydefinition the light is transmitted to the other orders, therebymaximizing the amount of light that is transmitted at an anglecorresponding to the first order transmission and above. In thisscenario, the first mode is approximately equal to the zero mode justbelow a fill factor of 0.25. Similarly, mode two is approximately equalto mode 0 at a fill factor just above 0.25.

FIG. 6 illustrates the amplitude of each mode's contribution to thefirst order transmission. As such, line 610 is the amplitudecontribution of the zero mode to the first order of transmission basedon the fill factor of the grating. Similarly, line 612 is the first modeamplitude contribution to the first order transmission based on the fillfactor. Line 614 is the second mode amplitude contribution to the firstorder transmission according to the fill factor. Similarly, line 616 isthe third mode and line 618 is the fourth mode amplitude contribution tothe first order transmission according to the fill factor. In this case,the amplitude of the first order transmission is necessarily enhanced bythe suppression of the zero order transmission.

FIG. 7 illustrates the amplitude of each mode's contribution to thesecond order transmission. As such, line 710 is the amplitudecontribution of the zero mode to the second order of transmission basedon the fill factor of the grating. Similarly, line 712 is the first modeamplitude contribution to the second order transmission based on thefill factor. Line 714 is the second mode amplitude contribution to thesecond order transmission according to the fill factor. Similarly, line716 is the third mode and line 718 is the fourth mode amplitudecontribution to the second order transmission according to the fillfactor. Under a fill factor of 0.5, the second order transmission andabove is quite small and has a minimal affect on the grating design.Particularly in the case of a photocell where an increased transmissionangle will increase the absorption of the photocell.

Now referring to FIG. 8, a graph is provided of the diffractionefficiency for each order of transmission with regard to the height ofthe grating. Line 810 is the diffraction efficiency of the zero ordertransmission according to the grating height. Line 812 is thediffraction efficiency of the first order transmission based on theheight of the grating. Further, line 814 is the diffraction efficiencyof the second order transmission based on the height of the grating. Thegraph in FIG. 8 is based on a fill factor of 0.25, where the zero modeamplitude contribution to the zero order transmission is approximatelyequal to the first mode contribution to the zero order transmission. Assuch, it can be seen that the zero order transmission is approximatelyequal to 0 at a grating height of about 1.35λ. Further, it can be seenthat the first order transmission denoted by line 812 is maximized atabout the same value. While the second order transmission 814 variesbased on the height, it remains a comparatively small value with respectto the diffraction efficiency of the first order transmission of thegrating.

FIG. 9 shows the amplitude contribution of each mode to the zero ordertransmission based on the angle of incidence relative to the grating. Inthis instance, an angle of 0 would be normal to the grating surface.Line 910 represents the minimum amplitude contribution of the zero modeto the zero order transmission. Similarly, line 912 illustrates themaximum amplitude contribution of the first mode to the zero ordertransmission based on the angle of incidence. As noted with regard toFIG. 5, ideally the zero mode amplitude contribution to the zero ordertransmission would be approximately equal to either the first or secondmode amplitude contribution to the zero order transmission. As such, thephases of the first or second mode could be manipulated to cancel outthe zero mode contribution. However, the amplitude of the zero modeincreases while the amplitude of the first mode decreases as the angleof the first order transmission increases. This implies that it is hardto suppress the normal direction of light where n=0 with an increasingfirst order transmission angle.

FIG. 10 is a graph of the diffraction efficiency for each order oftransmission relative to the angle of the first order of transmission.Line 1010 is the diffraction efficiency of the zero order transmissionaccording to the angle of the first order transmission. Line 1012 is thediffraction efficiency of the first order transmission based on theangle of the first order of transmission. Further, line 1014 is thediffraction efficiency of the second order transmission according to theangle of the first order of transmission. As can be understood from thisfigure, the light transmitted in the normal direction to the gratingincreases as the angle of the first order transmission increases.

FIG. 11 is a graph illustrating the difference of amplitude contributionto the zero order transmission between the zero mode and the first orsecond mode with respect to the fill factor of the grating. In addition,the transmission of light may be analyzed for s-polarized andp-polarized light as well. Line 1110 is the amplitude difference betweenthe zero mode and first mode that contribute to the zero ordertransmission for p-polarization. Line 1112 is the amplitude differencebetween the zero and second mode that contribute to the zero ordertransmission for p-polarization. As such, line 1110 is illustratedbetween a fill factor of 0 and 0.25, while line 1112 is illustratedabove 0.25 fill factor. Further, line 1114 is the amplitude differencebetween the zero mode and first mode contribution to the zero ordertransmission for the s-polarization. Line 1116 is the amplitudedifference between the zero and second mode contribution to the zeroorder transmission for s-polarization. A detailed review of FIG. 11illustrates that the minimum amplitude difference for s-polarization isa fill factor of 0.25 where the minimum amplitude difference forp-polarization is at approximately 0.34. Accordingly, a point may becalculated to provide the smallest amplitude difference for both s andp-polarization simultaneously. Here, a fill factor of 0.34 may beselected. The selected fill factor may be calculated based on an averageor a weighted average, although other methods may be used. For example,in this case the fill factor with the minimum average ofA_(d,m=0-2,n=0)(S) and A_(d,m=0-2,n=0)(P) may be used.

Now referring to FIG. 12, a graph of the effective refractive indexdifference with respect to the fill factor is provided. Line 1210 is theeffective refractive index difference between the zero and first ordertransmissions for s-polarization. Line 1212 is the effective refractiveindex difference between the zero and second order transmissions fors-polarization. Line 1214 is the effective refractive index differencefor the zero and first order transmissions for p-polarization, whileline 1216 is the effective refractive index difference between the zeroand second order transmissions for p-polarization. Again, the firstorder transmission is shown below a fill factor of 0.25 for s andp-polarizations, while the second order transmission is used for fillfactors above 0.25. Effective refractive index difference shows a closevalue for s and p-polarizations at the fill factor of 0.34, althoughthey are not exactly the same value. An average value of the effectiverefractive indexes for s and p-polarization at a fill factor of 0.34corresponds to a grating height of about 1.3λ.

FIG. 13 illustrates the diffraction efficiency with regard to the heightof the grating. Line 1310 is the diffraction efficiency for the zeroorder transmission for p-polarization. Line 1312 is the diffractionefficiency for the first order p-polarization. Line 1314 is thediffraction efficiency for the second order transmission forp-polarization with respect to grating height. Line 1316 is thediffraction efficiency for the zero order transmission fors-polarization with respect to grating height. Line 1318 is thediffraction efficiency for the first order transmission with respect tograting height for s-polarization. Line 1320 is the diffractionefficiency for the second order transmission for s-polarization withrespect to grating height. This result is provided for a period of 1.84θand a fill factor of 0.34 to provide minimum amplitude difference.Further, it is helpful to analyze this graph at a grating height of 1.3θcorresponding to the average of the effective refractive indexdifference. This may also correspond to the minimum average diffractionefficiency for the zero order transmission of s and p-polarization.Accordingly, the diffraction efficiency of the zero order transmissionfor the s-polarization is 2.4%. The diffraction efficiency for the firstorder transmission for s-polarization is 40.8%, while the second ordertransmission for s-polarization is only 6.7%. Similar results areobtained for p-polarization. The diffraction efficiency of the zeroorder transmission for p-polarization is 2.5%. The first orderdiffraction efficiency for the first order transmission is 45.3%.Further, the diffraction efficiency for the second order transmission isonly 2.4%.

FIG. 14 illustrates a grating with a first order transmission angle of40 degrees or greater. The transmission grating 1411 may have a base1416 of fused silica and protrusions 1418 formed of a dielectricmaterial. The protrusions 1418 may form a dielectric to air interface.As such, air surrounds the protrusions 1418 and is denoted by referencenumber 1414. The protrusions 1418 form grooves 1420 located between eachprotrusion 1418. The grooves 1420 may be filled with air 1414. Thegrating layer 1422 diffracts light directed towards the transmissiongrating 1411 from a light source into various diffraction modes.

Light may be provided to the transmission grating 1411, as denoted byarrow 1430. The light 1430 is provided from an angle generally normal tothe grating surface. In addition, the light 1430 may comprise variouslight polarizations. When the light 1430 interacts with the gratinglayer 1422, the incident light 1430 will form reflective componentsdenoted by R and transmissive components denoted by T.

The resulting characteristics of the reflective and transmissivecomponents are a factor of the refractive index (n) of the material, theperiod (p) of the grating, the fill factor (r) of the grating, and theheight (h) of the grating. The period of the transmission grating 1411is denoted by reference numeral 1424. The fill factor (r) is denoted byreference numeral 1426. The height (h) of the grating is the distancefrom the top of the protrusion 1418 to the bottom of the groove 1420,which is denoted by reference numeral 1428. In one embodiment, thegrooves 1420 and protrusions 1418 may be formed as right angles toresult in a rectangular grating. (i.e. having rectangular grooves andprotrusions). However, as one would readily understand, the grooves 1420and protrusions 1418 may not form exact right angles and variousprofiles may be used along the edge of the protrusions 1418.

For a solar cell assembly, an absorption layer 1450 may be coupled tothe base 1416. As such, the base 1416 may adjoin or be formed on theabsorption layer 1450 creating a direct interface to transfer the lightenergy from the grating 1411 to the absorption layer 1450. Further,another grating having similar properties to grating 1411 may be coupledto the opposite side of the absorption layer 1450 thereby redirectinglight transmitted toward the opposite side of the absorption layerhorizontally within the absorption layer 1450.

FIG. 15 illustrates a grating with a first order transmission angle ofabout 50 degrees. The transmission grating 1511 may have a base 1516 offused silica and protrusions 1518 formed of a TiO₂. The protrusions 1518may form a TiO₂ to air interface. As such, air surrounds the protrusions1518 and is denoted by reference number 1514. The protrusions 1518 formgrooves 1520 located between each protrusion 1518. The grooves 1520 maybe filled with air 1514. The grating layer 1522 diffracts light directedtowards the transmission grating 1511 from a light source into variousdiffraction modes.

Light may be provided to the transmission grating 1511, as denoted byarrow 1530. The light 1530 is provided from an angle generally normal tothe grating surface. In addition, the light 1530 may comprise variouslight polarizations. When the light 1530 interacts with the gratinglayer 1522, the incident light 1530 will form reflective componentsdenoted by R and transmissive components denoted by T.

The resulting characteristics of the reflective and transmissivecomponents are a factor of the refractive index (n) of the material, theperiod (p) of the grating, the fill factor (r) of the grating, and theheight (h) of the grating. The period of the transmission grating 1511is denoted by reference numeral 1524. The fill factor (r) is denoted byreference numeral 1526. The height (h) of the grating is the distancefrom the top of the protrusion 1518 to the bottom of the groove 1520,which is denoted by reference numeral 1528. In one embodiment, thegrooves 1520 and protrusions 1518 may be formed as right angles toresult in a rectangular grating. (i.e. having rectangular grooves andprotrusions) However, as one would readily understand, the grooves 1520and protrusions 1518 may not form exact right angles and variousprofiles may be used along the edge of the protrusions 1518.

For a solar cell assembly, an absorption layer 1550 may be coupled tothe base 1516. As such, the base 1516 may adjoin or be formed on theabsorption layer 1550 creating a direct interface to transfer the lightenergy from the grating 1511 to the absorption layer 1550. Further,another grating having similar properties to grating 1511 may be coupledto the opposite side of the absorption layer 1550 thereby redirectinglight transmitted toward the opposite side of the absorption layerhorizontally within the absorption layer 1550.

FIG. 16 illustrates a graph of the amplitude of mode 1 and mode 0 withrespect to the refractive index for the grating shown in FIG. 14. Line1610 is the amplitude of mode 0 with respect to the refractive index.Line 1612 is the amplitude of mode 1 with respect to the refractiveindex. The minimum amplitude of m=0 decreases as the refractive indexincreases. The maximum amplitude of m=1 increases as the refractiveindex increases. This implies that light received at an angle normal tothe grating (n=0) can be better suppressed by increasing the refractiveindex, thereby resulting is a small amplitude difference between mode 0and mode 1.

FIG. 17 provides a graph illustrating the diffraction efficiency withrespect to refractive index of the zero order transmission (normal tothe grating) and the first order transmission for the grating in FIG.14. Line 1710 is the diffraction efficiency with respect to therefractive index for the zero order transmission (normal to thegrating). Line 1712 is the diffraction efficiency with respect to therefractive index for the first order transmission (40 degrees for thegrating in FIG. 14). As can be noted from the graph the diffractionefficiency of the zero order transmission decreases as the refractiveindex increases. This is a result of mode 0 and mode 1 having nearlyequal amplitudes with a 180 degree phase shift.

FIG. 18 provides a graph illustrating the diffraction efficiency withrespect to the grating height for the zero order transmission and thefirst order transmission for the grating in FIG. 14. Line 1810 is thezero order transmission with respect the grating height. Line 1812 isthe first order transmission with respect to the grating height. As thefirst order transmission peaks, the zero order transmission has adiffraction efficiency near zero. Both the zero and first ordertransmission oscillate, but they are out of phase by 180 degrees. Thisresult is provided for a refractive index of n_(y)=2, a period ofp=1.07λ, a fill factor r=0.22, and grating height of h=0.59λ. In thisconfiguration the transmission efficiency of the zero order transmissionis T_(n=0)=0.9% at θ_(t,0)=0 degrees. In addition, the first ordertransmission is T_(n=1)(T_(n=−1))=48.8% at an angle ofθ_(t,1)=(θ_(t,−1))=40 degrees.

Further, FIG. 19 provides a graph illustrating the diffractionefficiency with respect to the grating height for the zero ordertransmission and the first order transmission for the grating in FIG.15. Line 1910 is the zero order transmission with respect the gratingheight. Line 1912 is the first order transmission with respect to thegrating height. As the first order transmission peaks the zero ordertransmission has a diffraction efficiency near zero. Both the zero andfirst order transmission oscillate but are out of phase by 180 degrees.This result is provided for a refractive index of n_(y)=2.38 (TiO₂ at600 nm), a period of p=0.9λ, a fill factor r=0.29, and grating height ofh=0.28λ. However, a grating period p=0.87λ-0.93λ, a fill factorr=0.24-0.34, and a grating height h=0.23λ-0.33λ may also be used. Inthis configuration, the transmission efficiency of the zero ordertransmission is T_(n=0)=0.7% at θ_(t,0)=0 degrees. In addition, thefirst order transmission is T_(n=1)(T_(n=−1))=49.5% at an angle ofθ_(t,1)=(θ_(t,−1))=50 degrees.

FIG. 20 illustrates one embodiment of a solar cell assembly 2012. Theassembly 2012 includes a first grating layer 2014, an absorption layer2022, and a second grating layer 2020. The absorption layer 2022 issandwiched between the first grating layer 2014 and the second gratinglayer 2020. Light 2010, for example from the sun, is received onto theassembly 2012. Direct sun light is generally received normal to manysurfaces, such as the tops of roofs or cars. However, light normal tothe absorption layer 2022 is not absorbed with as much efficiency aslight traveling in plane with the absorption layer 2022.

The first grating layer 2014 is made up of alternating portions firstand second portions. The first portion 2016 may be a first losslessdielectric formed directly on the absorption layer 2022 and the secondportion 2018 may be a second lossless dielectric formed directly on theabsorption layer 2022. The first lossless dielectric may have apermittivity ∈_(a,r) of about 2.25. The second lossless dielectric mayhave a permittivity ∈_(b,r) of about 6.25. In one embodiment the firstportion 2016 may be fused silica and the second portion 2018 may beTiO₂. Further, the absorption layer may have a permittivity ∈_(c,r) ofabout 16+j0.1. As discussed above, the first lossless dielectricalternates with the second lossless dielectric across the grating layer2014. The alternating portions diffract the light in the same manner asthe ridges and grooves of the grating in FIGS. 1 and 2. Each of theportions of first and second lossless dielectric may have a width ofabout 0.16λ, as denoted by arrow 2024. As such, the period of thegrating is 0.32λ, as denoted by arrow 2026. Further, the thickness ofthe first grating layer 2014 and the second grating layer 2020 may be0.12 A, as denoted by arrows 2028 and 2032, respectively. Further, thethickness of the absorption layer 2022 may be 3/32 A, as denoted byarrow 2030. When the absorption layer has a half guided wavelength((½)*(λ/4)=(⅛)λ), it has resonance. As such, a thin absorption layerwith a thickness less than (⅛)λ may be preferable. Further, adding thelossless dielectric at the top and bottom sides (for example the secondlossless dielectric 2018 with ∈_(b,r)=6.25 and thickness of 0.04λ mayalso provide resonance). The results of this can be seen in FIGS. 25 and26.

FIG. 25 shows a graph of transmittance with respect to normalizedwavelength for an absorption layer having a thickness less that (⅛)λ andan absorption layer sandwiched between two uniform dielectric layers.Line 2510 illustrates the transmittance with respect to normalizedwavelength for an absorption layer by itself having a thickness lessthat (⅛)λ. Line 2512 illustrates the transmittance with respect tonormalized wavelength for an absorption layer sandwiched between twouniform dielectric layers (for example ∈_(b,r)=6.25 and thickness of0.04λ). Further, FIG. 26 shows a graph of absorption with respect tonormalized wavelength for an absorption layer having a thickness lessthan ⅛λ (e.g. 3/32 λ) and an absorption layer sandwiched between twodielectric layers. Line 2610 illustrates the absorption with respect tonormalized wavelength for an absorption layer by itself having athickness less than ⅛λ. Line 2612 illustrates the absorption withrespect to normalized wavelength for an absorption layer sandwichedbetween two dielectric layers (for example ∈_(b,r)=6.25 and thickness of0.04λ).

The absorption layer by itself has a resonance at 0.75λ. thatcorresponds to a half guided wavelength. The dielectric layersandwiching method provides a resonance at λ. For both cases, theabsorption peaks with about 4% at resonance.

FIG. 21 is a graph illustrating the transmittance of the s polarizationand p-polarization with respect to the normalized wavelength of incidentlight for the assembly in FIG. 20. Line 2110 is the transmittance ofS-polarized light with respect to the normalized wavelength. Line 2112is the transmittance of P-polarized light with respect to the normalizedwavelength. Similarly, FIG. 22 is a graph illustrating the absorption ofthe s polarization and p-polarization with respect to the normalizedwavelength for the assembly in FIG. 20. Line 2210 is the absorption ofS-polarized light with respect to the normalized wavelength. Line 2212is the absorption of P-polarized light with respect to the normalizedwavelength. As can be seen, strong absorption peaks are present for bothS-polarization and P-polarization. S-polarization has an absorption peakof about 50% around A. P-polarization has an absorption peak of about30% around 0.8λ.

FIG. 23 is a side view of the assembly in FIG. 20 illustrating themagnetic field for S-polarization for two periods. The magnetic field isshown for A. The orientation of each arrow represents the localizeddirection of the magnetic field, while the size of the arrow representsthe magnitude of the magnetic field. A strong magnetic field is observedin the absorption layer 2022, due to resonance.

FIG. 24 provides one embodiment of a grating assembly that may be usedon the opposite side of an absorption layer from the incident light. Assuch, any of the aforementioned transmission gratings may be placed onthe side of the absorption layer receiving the light and the gratingdescribed below may be placed on the opposite side of the absorptionlayer. The reflective grating 2411 may be a fused silica transmissiongrating with a silica to air interface. As such, air surrounds a silicastructure 2412 and is denoted by reference number 2414. The silicastructure 2412 includes a base 2416 of solid fused silica. Fused silicais very transparent and transmits a very broad bandwidth of light.Further, fused silica offers a very stable material that can be usedover a wide range of temperature conditions. In addition, fused silicagratings may be easily etched to provide the grating properties requiredfor many applications. Fused silica has an index of refraction of about1.45 in contrast to air with an index of refraction of about 1. Thesymbol n_(α) is used to denote the refraction index of air and n_(β) isused to denote the refraction index for fused silica.

Protrusions 2418 extend from and are integral with the base 2416. Beingintegral with the base 2416 the protrusions 2418 are also formed offused silica. The protrusions 2418 form grooves 2420 located betweeneach protrusion 2418. The grooves 2420 may be filled with air 2414,thereby providing an air fused silica interface across the grating layer2422. The grating layer 2422 diffracts light directed towards thereflection grating 2411 from a light source into various diffractionmodes.

Light may be provided to the grating 2411, as denoted by arrow 2430. Thelight 2430 is provided from an angle generally normal to the gratingsurface. In addition, the light 2430 may comprise various lightpolarizations. For example, the incident light may comprise componentsthat are s-polarized and p-polarized. When the light 2430 interacts withthe grating layer 2422, the incident light 2430 will form reflectivecomponents denoted by R.

The resulting characteristics of the reflective and transmissivecomponents are a factor of the refractive index (n) of the material, theperiod (p) of the grating, the fill factor (r) of the grating, and theheight (h) of the grating. The period of the grating is the distancefrom the start of one groove to the start of the next groove. The periodof the transmission grating 2411 is denoted by reference numeral 2424.The fill factor (r) can be defined as the ratio of the ridge width orgroove width to the period of the grating, which is denoted by referencenumeral 2426. The height (h) of the grating is the distance from the topof the protrusion 2418 to the bottom of the groove 2420, which isdenoted by reference numeral 2428. In one embodiment, the grooves 2420and protrusions 2418 may be formed as right angles to result in arectangular grating. (i.e. having rectangular grooves and protrusions)However, as one would readily understand, the grooves 2420 andprotrusions 2418 may not form exact right angles and various profilesmay be used along the edge of the protrusions 2418. As such, thedefinition for the fill factor (r) or grating height (h) may be slightlymodified depending on the shape of the projections 2418 and grooves2420. As such, these values may be determined based on the center ofgravity of the projections 2418 and grooves 2420.

In addition, the base 2416 of fused silica may be formed on top of asandwich structure 2450. The sandwich structure 2450 includes a firstlayer 2452 of titanium dioxide, a layer of fused silica 2454, and asecond layer 2456 of titanium dioxide. The first layer 2452 beinglocated between the base 2416 and the layer of fused silica 2454. Thelayer of fused silica 2454 being located between the first layer 2452and the second layer 2456. The base 2416 and the sandwich structure 2450forming a distributed Bragg reflector located below the grating.

FIGS. 27 and 28 illustrate another embodiment of a grating. Thisembodiment may be used for a solar cell, but may also be used forcoupling. In this embodiment, the assembly 2710 includes a substrate2712 and a grating 2714. The grating may correspond to any of thepreviously described gratings, for example the grating of FIG. 15. Inthis instance, the grating width is selected to be smaller than thedistance where the light is reflected back to the top surface of thesubstrate. This can be better understood in reference to arrows 2716that represent the resulting direction of light travel through thesubstrate due to the grating diffraction. The light is primarilydirected to the first order transmission at a 50 degree angle. If theangle is over about 43.6 degrees, the light energy will be internallyreflected and propagate horizontally within the substrate. Therefore,arrows 2716 show light bouncing between the top and bottom surface ofthe substrate as it propagates horizontally.

Further, it is helpful to note that for a coupling application thegrating width can be important. For example, if the light diffracted bythe grating reflects internally within the substrate and returns to thegrating, the unabsorbed portion of the light may exit the substrate dueto the grating. As such, the grating width is selected to be smallerthan twice the distance required for the internally reflected light totravel from the top surface to the bottom surface and back to the topsurface. More specifically, the width of the grating w_(g) is less thantwice the thickness of the substrate t_(s) times the tangent of thefirst order transmission angle θ_(t,1). (w_(g)<2t_(s) (tan(θ_(t,1)))).

One specific example is shown in FIG. 28. The width of the beam 2810provided to the assembly is 5λ, as denoted by line 2812. The width ofthe grating is selected to be 9.9λ (0.9λ by 11 periods). Where the beamtravel is 5λ, as denoted by line 2818 and the substrate height is 5λ, asdenoted by line 2816. If the grating were allowed to extend beyond thewidth where the internally reflected light reaches the top surface, thegrating would allow much of the light to exit the substrate therebygreatly reducing absorption efficiency.

As a person skilled in the art will readily appreciate, the abovedescription is meant as an illustration of the principles thisapplication. This description is not intended to limit the scope orapplication of the invention in that the invention is susceptible tomodification, variation and change, without departing from spirit of theinvention, as defined in the following claims.

We claim:
 1. A solar cell assembly comprising: an absorption layer forabsorbing light and converting the light into electrical energy; a firstgrating layer formed by alternating portions of a first losslessdielectric formed directly on the absorption layer and a second losslessdielectric formed directly on the absorption layer, the first gratinglayer having a thickness of about 0.12λ and a period of about 0.32λ; anda second grating layer disposed on a second side of the absorptionlayer, the second side being opposite the first side, the second gratinglayer having a thickness of about 0.12λ and a period of about 0.32λ;wherein the first grating layer, the second grating layer, and theabsorption layer form a sandwich structure, the absorption layer beinglocated between the first layer and the second layer.
 2. The solar cellassembly according to claim 1, wherein the second grating layercomprises lossless dielectrics.
 3. The solar cell assembly according toclaim 1, wherein the second grating layer is formed by alternatingportions of the first lossless dielectric and the second losslessdielectric.
 4. The solar cell assembly according to claim 3, wherein thefirst lossless dielectric has a different permittivity than the secondlossless dielectric.
 5. The solar cell assembly according to claim 4,wherein the first lossless dielectric has a permittivity of about 2.25and the second lossless dielectric has a permittivity of about 6.25. 6.The solar cell assembly according to claim 1, wherein the absorptionlayer has a thickness of less than ⅛ a wavelength range of light (λ). 7.The solar cell assembly according to claim 6, wherein the absorptionlayer has a thickness of about 3/32λ.
 8. The solar cell assemblyaccording to claim 3, wherein the first portion comprises fused silicaand the second portion comprises titanium dioxide.